1. Field of the Invention
The present invention relates to a chrominance signal demodulation circuit for separating a chrominance signal into a red color difference signal (R-Y) and a blue color difference signal (B-Y).
2. Description of the Related Art
Generally, in a conventional video magnetic recording/reproducing apparatus, a frequency of a chrominance signal of a composite video signal is down-converted, and is recorded on a magnetic tape together with a luminance signal of the composite video signal. In playback, the down-converted chrominance signal is restored to have its original frequency by an up-converter of the apparatus.
Recently, in the video magnetic recording/reproducing apparatus, there is employed an FM modulated chrominance signal reproduction processing system capable of compensating a time-base error or a color noise reduction, wherein the chrominance signal play backed and up-converted is separated into a red color difference signal (R-Y) and a blue color difference signal (B-Y). After the respective color difference signals (R-Y), (B-Y) are appropriately processed, they are again added to each other to restore original chrominance signals.
FIG. 7 is a block diagram of a chrominance signal demodulation circuit for separating an up-converted chrominance signal into a red color difference signal (R-Y) and a blue color difference signal (B-Y) in the prior art.
Referring to FIG. 7, a chrominance signal from an input terminal 1 is applied to a color burst discriminator (separator) 2, where only its color burst signal is discriminated or separated from the chrominance signal so that the separated color burst signal is fed to a phase comparator 3.
The phase comparator 3 compares a phase of the color burst signal with a phase of a color subcarrier signal generated from a color subcarrier generator 5, and outputs an error signal responsive to an amount of a phase difference therebetween. This error signal is inputted to a voltage-controlled oscillator (referred to as VCO) 5a of the color subcarrier generator 5 through a low-pass filter (LPF) 4.
The VCO 5a oscillates a signal having a frequency responsive to a level of the error signal outputted from the low-pass filter 4. From the color subcarrier generator 5, two signals are outputted respectively having phases of cos xcex8 and sin xcex8, the latter is produced from the cos xcex8 signal by a 90xc2x0-phase shifter 5b. The two signals are respectively inputted to multipliers 7 and 6 as the color subcarrier signals. The multiplier 6 produces a red color difference signal (R-Y) and its harmonic components having a double frequency thereof by multiplying the chrominance signal fed from the input terminal 1 and the color subcarrier signal (sin xcex8). The multiplier 7 produces a blue color difference signal (B-Y) and its harmonic components having a double frequency thereof by multiplying the chrominance signal fed from the input terminal 1 and the color subcarrier signal. (cos xcex8).
The respective harmonic components are removed by low-pass filters 8 and 9, resulting in the red color difference signal (R-Y) and the blue color difference signal (B-Y), respectively.
As mentioned in the above, in the chrominance signal. demodulation circuit in the prior art, the chrominance signal is inputted to the multipliers 6, 7, and either of the red and blue color difference signals (R-Y), (B-Y) is obtained by multiplying the color subcarrier signal (sin xcex8 or cos xcex8) which has an identical phase to that of the color burst signal of the chrominance signal. However, when the phase of the color subcarrier signal (cos xcex8 or sin xcex8) does not completely accord with that of the color burst signal, a precise chrominance signal demodulation processing can not be expected.
In the chrominance signal demodulation circuit mentioned above, the phase comparator 3 and the VCO 5a are typically composed of an analogue circuit, respectively.
The phase comparator 3 and the VCO 5a each composed of such an analogue circuit are apt to be affected by a noise. In addition, they have inherent production dispersion in the characteristics. Thus, in order to alleviate the effect of the noise and the dispersion of the characteristics, the error signal outputted from the phase comparator 3 needs to be inputted to the VCO 5a through the low-pass filter 4 having a gain of less than 1.
However, in the event of employing a filter having a certain time constant in a loop circuit, a response of a feedback loop delays. Thus, it takes some time to make the phase of the color subcarrier signal outputted from the subcarrier generator 5 accord with that of the color burst signal of the inputted chrominance signal.
Accordingly, when an abrupt change of the phase in the inputted chrominance signal develops due to such a tape track skew error developed at a playback in the magnetic recording/reproducing apparatus, it takes some time equivalent to several to several tens horizontal line periods till the phase of the color subcarrier signal accords with that of the color burst signal, resulting in a problem of a phase shift during the period.
And, after these red and blue color difference signals (R-Y), (B-Y) having the phase shift are processed with a time-base error compensation or a color noise reduction, they are subjected to a color reproduction process by addition for displaying a color image on a TV receiver (not shown). As a result, a color shift appears at an upper portion of the image displayed on the TV receiver.
Accordingly, a general object of the present invention is to provide a chrominance signal demodulation circuit used in, for instance, a video magnetic recording/reproducing apparatus where the above disadvantages have been eliminated.
A specific object of the present invention is to provide a chrominance signal demodulation circuit comprising: multiplying means for producing a first and second color difference signals by multiplying a color subcarrier signal and a color burst portion of a chrominance signal inputted to the chrominance signal demodulation circuit; phase error producing means for outputting a phase error signal from the first and second color difference signals by calculating a phase error between a phase of the color burst portion of the inputted chrominance signal and phases the color subcarrier signals; phase compensation means for compensating the phase of the color subcarrier signal so as to accord with the phase of the color burst portion in responsive to the phase error signal generated from the phase error producing means, and means for replacing the respective color burst portions of the first and second color difference signals with a predetermined standard color burst signal.
Another and more specific object of the present invention is to provide a method for demodulating a chrominance signal inputted to a chrominance signal demodulation circuit, the method comprising the steps of: producing first and second color difference signals from the chrominance signal by multiplying a color burst portion of the chrominance signal and color subcarrier signals locally generated from a color subcarrier generation means, the color subcarrier signals having a phase difference of 90xc2x0 degrees each other and a frequency of the chrominance signal inputted to the chrominance signal demodulation circuit; producing a phase error signal representing a phase error between a phase of the color burst portion of the chrominance signal and a phase of one of the first and second color difference signals, by detecting a level difference between respective color burst portions of the first and second color difference signals and calculating the phase error from the level difference using a conversion table; compensating the phase error developed in the color subcarrier generation means so as to accord a generated phase of one of 2 color subcarrier signals with the phase of the color burst portion of the chrominance signal inputted to the chrominance signal demodulation circuit by feeding the phase error signal to the color subcarrier generation means, and repeating a process of compensating the phase error for every horizontal period of the chrominance signal; and replacing color burst portions of the first and second color difference signals with a predetermined standard color burst signal after undergoing the process of phase error compensation.